1. Field of the Invention
The present invention relates to a hybrid ferromagnet/semiconductor spin device.
2. Description of the Conventional Art
Since a transistor replacing a vacuum tube was developed at the Bell laboratory in 1948, an electron device technique based on a complimentary metal oxide semiconductor (CMOS) technique has been greatly developed by the Moore's law (a law that an amount of data that can be stored in a micro chip is increased twice per 18 months). However, due to a technical progress over the past 40 years, a current electron device technique which has faithfully followed the Moore's law is already in a saturated state, and it is anticipated that a device fabrication will be impossible after 20 years since a size of a semiconductor reaches a silicon grating constant (approximately 0.54 nm). According to this, a new next generation device is fast required.
To this end, a development of a metal oxide semiconductor field effect transistor (MOSFET) having a gate of a nano size is being performed in a hurry by deepening the CMOS technique using a nano technique, and a memory and a logical device using a single electron transistor (SET) and a quantum dot are being developed in haste. Also, recently, a thin film transistor using a high molecule was developed thus to prove that several logical circuits can be made by a high molecular thin film. According to this, various molecular electronics devices are being developed by a molecular adjustment. A field of the molecular electronics device is for developing a next generation electronics device using a new material differently from a current electron device technique based on a semiconductor.
So far, in a semiconductor electronics device, only a charge of a carrier (an electron having a negative charge and a hole having a positive charge) has been controlled by an electric field without considering a spin between the charge and the spin, two characteristics of an electron. At the background of a silicon semiconductor industry, some scientists have tried to implement a spin-dependent electron transport. As the result, a new paradigm, a spintronics (a mixed word of a spin and electronics) technique for developing an electronics device by considering the charge of the electron and a degree of freedom the spin is being spotlighted in a recent science technique field. According to this, it is anticipated that the next generation electron device will be greatly developed with a later nano technique development since the spin electron device has characteristics such as an ultra high speed, a ultra low power, and etc. with non-volatility when compared with the conventional electron device.
As a representative example, a reproducing head uses a giant magneto-resistance (GMR) phenomenon generated at a stacked structure between a ferromagnetic metal and a paramagnetic metal and discovered in 1988. The giant magneto-resistance was first discovered in a Fe/Cr multi-layer thin film, and the reason is because a fabrication of a multi-layer thin film of a uniform nano thickness (less than 1 nm) having no defection becomes possible since a ultra high vacuum (UHV) technique advances. The GMR phenomenon was first applied to a substantial device since a spin-valve structure that a magnetization of each magnetic layer is performed freely and independently by inserting a non-magnetic metal layer such as Cu and etc. between two ferromagnetic metal layers was developed.
In case of the spin-valve structure, a thickness of an inserted non-magnetic metal layer is thick enough to remove magnetic defects of two adjacent ferromagnetic metal layers. Thereby, the spin-valve structure can be sensitively reacted to a very small external magnetic field corresponding to several Oe. One of the most conspicuous progresses in the spintronics technique after the GMR discovery is a tunneling phenomenon of a spin-polarized electron (a tunneling magneto-resistance phenomenon) which was observed in a thin film structure of a magnet/insulator/magnet at a high temperature. Even if a low temperature tunneling magneto-resistance phenomenon was discovered approximately 30 years ago, a room temperature tunneling magneto-resistance phenomenon that is very important in an application aspect was discovered in 1995.
The greatest concern of the spintronics research is to implement a memory and a logical transistor by considering a charge and a degree of freedom of a spin at the same time. A research for a spin injection that a spin-polarized electron is injected from a ferromagnetic metal to a paramagnetic metal has been partially performed. It was reported that the spin injection is caused by an interesting phenomenon such as a spin accumulation and etc. In 1993, a bipolar spin transistor was fabricated as a spin switch storage device which has a structure that a paramagnetic metal such as Au is inserted into two ferromagnetic metals, in which a spin is injected into the paramagnetic metal by using one ferromagnetic metal as a spin source and then detecting the injected spin by another ferromagnetic metal. Even though the spin transistor composed of a metal has proved the spin injection phenomenon experimentally, it has a limitation to be used as a memory device due to a small impedance.